1. Field of the Invention
The invention relates to a dark ring of a microdisplay and its driving method, in order to prevent the fringe effect occurred between the dark ring and the adjoining pixels within the microdisplay such as liquid-crystal-on-silicon (LCOS) display.
2. Discription of the Prior Art
LCOS display is one of the most popular microdisplays that are broadly adopted in large-sized TV. Because of mechanical and process limitations of the LCOS display, it is necessary to place a dark ring 110 in peripheral of the LCOS display 100 by using a top metal layer, as shown in FIG. 1.
Referring to FIG. 2, in a normally-white mode, there are two voltages, Vτ1 and Vτ10, at which liquid crystal contained within pixels will be biased to obtain a zero transmission rate. It suggests we can bias the liquid crystal at either Vτ1 or Vτ10 to show an all-black image. But the liquid crystal cannot be biased by DC voltages for a long time to avoid undesired deformation. Therefore, the dark ring is required to alternately bias at voltage Vτ1 and Vτ10 on a frame basis. Usually, it is called negative and positive polarity when biased at Vτ1 and Vτ10.
In the normally-white mode, when the LCOS display shows an all-black image with all pixels within the LCOS display changing from negative to positive polarity (that is, in case of frame inversion) and with gate drivers of the LCOS display scanning from top to bottom, the voltage of the dark ring will also change from Vτ1 to Vτ10 at time t0 when activating the topmost scan line of the LCOS display. In this case, data voltages for the adjoining pixels on each scan line (from top to bottom) within the LCOS display will sequentially change from Vτ1 to Vτ10 at time t0, t1, and tn. It means there will be a constant voltage difference between the dark ring and the adjoining pixels on the lower scan lines within the LCOS display for almost a whole frame period, and the fringe effect (bright lines) between the dark ring and the adjoining pixels on the lower scan lines within the LCOS display will occur due to the constant voltage difference and decrease the quality of the image.
This also applies when the LCOS display shows an all-black image with all pixels within the LCOS display changing from negative to positive polarity (that is, in case of frame inversion) and with gate drivers of the LCOS display scanning from bottom to top. Furthermore, this also applies when the LCOS display is performing line inversion or dot inversion.
Therefore, there is a need to provide a new dark ring of the LCOS display and its driving method to prevent the fringe effect and to increase the quality of images.